High voltage pulsed power supply using solid state switches with droop compensation

ABSTRACT

Systems and methods for generating a high voltage pulse. A series of voltage cells are connected such that charging capacitors can be charged in parallel and discharged in series. Each cell includes a main switch and a return switch. When the main switches are turned on, the capacitors in the cells are in series and discharge. When the main switches are turned off and the return switches are turned on, the capacitors charge in parallel. One or more of the cells can be inactive without preventing a pulse from being generated. The amplitude, duration, rise time, and fall time can be controlled with the voltage cells. Each voltage cell may also includes a balance network to match the stray capacitance seen by each voltage cell. Droop compensation is also enabled.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/838,600 filed May 4, 2004 and entitled, HIGH VOLTAGE PULSEDPOWER SUPPLY USING SOLID STATE SWITCHES, which application isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to systems and methods for generating highvoltage pulses. More particularly, the present invention relates tosystems and methods for generating high voltage pulses controlled bysolid state switches.

2. Background and Relevant Art

Many applications need a pulsed power supply that is able to deliverhigh voltage pulses. Spectrometers, klystrons, accelerators, radartransmitters, high impedance electron guns, ion tubes, liquid polarizingcells, etc., are examples of applications that need high voltage pulses.In conventional systems, a pulsed power supply uses a high voltage pulseforming network and some sort of switch such as a spark gap or athyratron.

These types of pulsed power supplies are often created using principlesof Marx Generators. Generally, a Marx Generator is circuitry thatgenerates a voltage pulse by charging a group of capacitors in paralleland then discharging the capacitors in series. FIG. 1 illustrates anexample of a typical Marx Generator. In FIG. 1, a charging voltage 101is applied to a pulse forming network 100. The stage capacitors 104charge through the resistors 102 in a parallel fashion. The spark gaps106 prevent the capacitors 104 from discharging into a load 108 untilcertain conditions are satisfied.

When the capacitors 104 are sufficiently charged, the lowest gap istypically allowed to break down or is triggered. When the lowest gapbreaks down or triggers, two capacitors are effectively in series andthe next gap breaks down. Very quickly, all of the gaps break down. Theresult of this process is that the capacitors 104 are connected inseries and a voltage pulse is generated and delivered to the load 108.The capacitors 104 of a Marx Generator may also be charged usinginductors or a series of transformers. In other example, the resistors102 are replaced with inductors. The spark gaps can alternatively bereplaced, for example, with switches such as thyratrons.

Because a Marx Generator is charged in parallel, the magnitude of thevoltage pulse can be increased by adding additional charging sections.However, it has been found that the number of sections that can bestacked together is effectively limited by stray capacitance. As thenumber of sections in the pulse forming network increases, the straycapacitance to ground also increases. One of the effects of straycapacitance is that the current is diverted to ground. The straycapacitance also has an adverse affect on the rise time and/or falltimes of the voltage pulse. The stray capacitance therefore limits thenumber of sections that can be included in the pulse generator.

The stray capacitance can also have an impact on the voltage that aparticular section sees. In addition, the stray capacitance seen by onesection is usually different from the stray capacitance seen by anothersection of the Marx Generator. Because the stray capacitance is notbalanced across the sections of the pulse generator, some of thesections may experience higher voltages and may therefore malfunction.Although most systems are affected by stray capacitance, the inductors,resistors, transformers, and isolated supplied needed to charge thecapacitors in the pulse generator also add stray capacitance to thepulse generator. In other words, the components of conventional pulsegenerators introduce additional stray capacitance to the system andfurther reduce the number of sections that can be successfully connectedtogether.

Because Marx Generators are often used to generate high voltages, theycan be quite large in both size and weight. In addition, a MarxGenerator that generates hundreds of kilovolts should be using oil. Oilis typically necessary, but is often undesirable. Conventional pulsedpower supplies or Marx Generators are often large and expensive, arelimited by stray capacitance, and use components (such as thyratrons)that reduce their reliability.

BRIEF SUMMARY OF THE INVENTION

These and other limitations are overcome by embodiments of the presentinvention, which relates to systems and methods for generating a voltagepulse. In one embodiment of the invention, a series voltage cells withrelatively low voltage requirements can be stacked together in series,each voltage cell including a capacitor connected in series with aswitch (such as a solid state switch) that can be turned on and off.When multiple voltage cells are connected to form a pulse generator, thecapacitors of the voltage cells are charged in parallel and dischargedin series using one or more switches. Main switches are used at least todischarge the capacitors and return switches are used at least to chargethe capacitors.

When the voltage cells are stacked, for example, the capacitors and mainswitches are connected in series. The capacitors are isolated from eachother by the main switches which are turned off. When the main switchesare on, the capacitors are connected in series and a voltage pulse isgenerated. When the main switches are off, the return switches may beturned on and provide a return path for the current that charges thecapacitors in the voltage cells. Thus, the return switches are off whenthe main switches are on such that the capacitors discharge to the load.Advantageously, the capacitors can be charged without the use ofinductors, resistors, or isolated supplies, thereby reducing some of thestray capacitance associated with conventional Marx Generators. Inaddition, the switches can be driven by use of an auxiliary supplywithout using inductors, resistors, isolated supplies, or step downsupplies.

The capacitors in each voltage cell can be charged through a diodestring supply line. A return path for the charging current is providedthrough return switches. When the capacitors are charging or arecharged, main switches placed between successive capacitors are in anoff state and prevent the capacitors from discharging in series. Whenthe main switches are turned on, the capacitors are then connected inseries and discharge. During discharge, the return switches are turnedoff. To recharge the capacitors, the main switches are turned off andthe return switches are turned back on. The return switches can also beturned on during discharge to help, in one embodiment, decrease the falltime of the pulse by providing a path for the stray capacitance todischarge.

The voltage cells can also be configured to generate either a positiveor a negative voltage pulse. In one embodiment, a bipolar pulsegenerator has a capacitor bank that includes a series of voltage cellsconfigured to generate a positive pulse can be connected with acapacitor bank that includes a series of voltage cells configured togenerate a negative pulse. This bipolar pulse generator can charge allof the capacitors in both sets of voltage cells at the same time. Theswitches in the respective capacitor banks can be controlled todischarge one set of capacitors to generate either the positive or thenegative pulse. In addition, voltage cells that are configured to chargein series can be added to provide droop control and control the shape ofthe generated voltage pulse.

Each voltage cell may also includes a balance network that balances thestray capacitance seen by that voltage cell. Because each voltage cellin a series of voltage cells “sees” a different stray capacitance, thebalance networks can be adapted to match the stray capacitance seen bythe voltage cells. This has the benefit of balancing the voltage seen byeach cell.

The voltage cells can be used to adjust the voltage pulse by controllingwhich voltage cells are active. In other words, one or more of thevoltage cells can be made inactive to alter the voltage pulse withoutaffecting the ability to generate the voltage pulse. At the same time,the failure of a particular cell does not prevent the pulse generatorfrom pulsing. Thus, embodiments of the present invention can control theamplitude of the voltage pulse, a duration or width of the voltagepulse, the rise and fall times of the voltage pulse, and the like or anycombination thereof.

In one embodiment of a pulse generator, some of the voltage cells canfurther be configured to include a ringing circuit that can be used toprovide droop correction. The circuit can provide droop compensation ordroop correction to the output of the pulse generator such that thedroop correction is smooth rather than jagged or saw-toothed. Theringing circuit includes capacitors that charge in series and thendischarge in parallel to a ringing capacitor, which is able to dischargein a manner to provide smooth droop compensation or correction.

In another embodiment of the invention, the voltage cells provideisolation protection. For example, if a particular voltage cell fails,then that cell can be isolated without impacting the ability of thepulse generator to generate and deliver a pulse to a load. Each voltagecell typically includes a diode across the return switch. If the mainswitch is off, then the discharge current can flow through this diodeand the cell is effectively isolated.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by the practice of the invention. Thefeatures and advantages of the invention may be realized and obtained bymeans of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present inventionwill become more fully apparent from the following description andappended claims, or may be learned by the practice of the invention asset forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features of the invention can be obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 illustrates an example of a Marx Generator that uses spark gapsto generate a voltage pulse;

FIG. 2 illustrates one embodiment of a pulse generator that usesswitches to control a series of voltage cells;

FIG. 3A illustrates a series of voltage cells and illustrates a mainswitch used to connect the capacitors in the voltage cells in series andreturn switches that provide a return path for a charging current.

FIG. 3B is a more detailed diagram of one embodiment of a pulsegenerator and illustrates the path of the charging current for eachvoltage cell through a diode string supply and illustrates a diodestring to provide auxiliary power to the switch drives.

FIG. 4 illustrates one embodiment of a series of voltage cells arrangedto generate a positive voltage pulse;

FIG. 5 illustrates one embodiment of a series of voltage cells arrangedto generate a negative voltage pulse;

FIG. 6 illustrates an embodiment of a pulse generator that can generateboth positive and negative pulses;

FIG. 7 illustrates another embodiment of a pulse generator that includesvoltage cells arranged to provide droop control for the voltage pulse;

FIGS. 8A through 8D illustrate an embodiment of a portion of a pulsegenerator that provides droop control in a pulse generator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to systems and methods for generating avoltage pulse. Embodiments of the invention can control an amplitude ofthe voltage pulse, a duration or width of the voltage pulse, a rise timeof the voltage pulse, a fall time of the voltage pulse, and the like orany combination thereof. Some embodiments of the invention can generateand deliver a voltage pulse without the use of transformers.

Embodiments of the invention include voltage cells that typically haveboth a capacitor and a switch in series. The first and last voltagecells in a series of voltage cells may be adapted to connect to theload. Return switches are also included in most voltage cells. Thereturn switches provide a path for the charging current supplied througha diode chain or a diode chain supply line. Advantageously, the returnswitches eliminate the use of inductors, resistors, and isolatedsupplies prevalent in conventional pulse generators. The switch drivesare also provided with energy through an auxiliary diode chain, therebyeliminating the need for inductors, resistors, isolated supplies, andstep down supplies that would otherwise be needed to provide theauxiliary power to the switch drives. Also, the elimination of thesecomponents reduces the stray capacitance to ground associated with thesystems and methods described herein, which enables more voltage cellsor sections to be stacked in series.

FIG. 2 illustrates a block diagram of one embodiment of a pulsegenerator or system for generating and delivering a high voltage pulseto a load. More particularly, the system 200 generates and delivers ahigh voltage pulse to the load 206. In the system 200, a switchedcapacitor bank 202 includes one or more capacitor or voltage cells 210that are typically arranged in series. The voltage cells 210 are used tostore the energy that is delivered to the load 206 as a voltage pulse.

The voltage cells 210 are typically associated with switches 212 thatare controlled by the switch drivers 204. By controlling the controlsignals 208, the switch drivers 204 can turn the switches 212 on/off.The state of the switches 212, determines whether the voltage cells 210are charging or discharging through the load 206. In one embodiment, theswitches can be switched on and or off at particular times. The timingof the control signals 208 can alter the rise time of the voltage pulse,the fall time of the voltage pulse, and the like. Some embodiments ofthe invention also enable the waveform to be shaped or otherwisecontrolled.

In one embodiment, the effects of stray capacitance are reduced suchthat more voltage cells can be connected in series. Because more voltagecells can be connected in series, a lower voltage source can be used togenerate a larger voltage pulse. Also, the switch drivers can be ratedfor lower voltages. As a result, the cost and size of the pulsegenerator are typically reduced.

In one embodiment of the system 200, the voltage cells are charged inparallel and discharged in series by controlling the state of theswitches 212. One of the advantages of the system 200 is that one ormore of the voltage cells 210 can fail without preventing the system 200from delivering a high voltage pulse to the load 206. The system 200 canbe configured to deliver a positive voltage pulse, deliver a negativevoltage pulse, or deliver either a positive or negative voltage pulse(bipolar output). In addition, the control signals 208 can be used tocontrol a duration of the voltage pulse, a magnitude of the voltagepulse, a rise time of the voltage pulse, and the like or any combinationthereof. The control signals may be optically coupled to the switchdrivers 204 in one embodiment.

FIG. 3A illustrates a diagram of one embodiment of a system for storingand/or delivering a high voltage pulse to a load. More particularly,FIG. 3A illustrates a few voltage cells connected in series, but one ofskill in the art can appreciate the more or fewer voltage cells can beincluded. Each voltage cell is similarly configured and operate togetherto (i) charge the capacitors in parallel or independently of othervoltage cells and (ii) discharge the capacitors in series.

For example, the voltage cell 362 includes, in this example, a capacitor366 that is used to store a charge. At the same time, the capacitor 378in the voltage cell 374 is also storing a charge. When storing a charge,the switches 364 and 376 (and similar switches in other voltage cells)are off. Thus, the capacitors 366 and 378 can charge in parallel orindependently.

The capacitors 366 and 378 are charged by the supply line 388 andbecause the switches 364 and 376 are off, the return switches 368 and380 are turned on to provide a return path for the charging currentprovided through the supply line 388. As illustrated in FIG. 3B, thesupply line 388 is a diode string and typically includes one or morediodes to separate the voltage cells. The switch drives 370 and 382control the state of the switches 364 and 376, respectively. The switchdrives 372 and 384 control the state of the return switches 368 and 380,respectively. The control lines 390 can be used to control the states ofthe switches 364, 376 and the states of the return switches 368, 380.

When the switches 364, 376 are turned on and the return switches 368,380 are turned off, then the capacitors 366, 378 are connected anddischarge in series to the load 392. In other words, connecting anddischarging the capacitors 366, 378 in series generates a high voltagepulse that is applied to the load 392. Turning off the switches 364, 376can terminate the pulse. Thus, the duration of the pulse can becontrolled through controlling the switches 364, 376. If a particularvoltage cell is non-functional, the supply line 388 is an example of thepath that the current can follow during delivery of the pulse. In otherwords, a non-functional voltage cell does not prevent a pulse from beinggenerated or delivered to the load 392.

FIG. 3B illustrates one embodiment of a high voltage pulse generator.This embodiment includes three voltage cells, but as previously stated,one of skill in the art can appreciate that more or fewer stages can beincluded. In this example, the capacitors 310, 314, and 318 storecharge. Charge is stored by turning the switches 308, 312, and 316 to anoff state.

When charging the capacitors 310, 314, and 318, the return switches 332,334, and 336 are in an on state and the main switches 308, 312, and 316are off. The path 326 illustrates a path of the current from the powersupply 304 that charges the capacitor 318. At the same time, the powersupply 304 delivers current through the path 324 to charge the capacitor314. The path 324, after passing through the capacitor 314, proceedsthrough the return switch 336 via the connection 330. A similar paththrough the diode 320 and the return switches 334, and 336 is used tocharge the capacitor 310. The current that charges the capacitor 310proceeds through the connection 328 and then through the return switches334 and 336. The diodes 320 and 322 isolate the power supply 304 fromthe pulse and ensure that the current flows to the load 306 duringdischarge. At the same time, the diodes permit the pulse to pass aroundany voltage cell that is not functioning.

During discharge of the capacitors, the switches 308, 312, and 316 areturned on using the control signals provided to the switch drives 338,342, and 346, respectively. At the same time, the control signals aredelivered to the switch drives 340, 344, and 348 to turn the returnswitches 332, 334, and 336 off. When the return switches 332, 334, and336 are turned off, the discharge current does not flow through thereturn switches and is delivered to the load 306.

As illustrated in FIG. 3B, the connection 328 is shown as a wire orshort while the connection 330 is illustrated as an inductor. Typically,all of the connections in the voltage cells are the same, but two typesof connections are illustrated in this example to describe additionalembodiments of the invention. When the connection is an inductor likethe connection 330, the timing between turning the switch 316 on and thereturn switches off can be delayed. An inductive connection 330 canincrease the rise time of the leading edge of the pulse.

For example, when the switches 308, 312, and 316 are turned on and thereturn switches 332, 334, and 336 are also on, a current begins to buildin the inductive connections like the connection 330. After allowing theinductance to build, the return switches 332, 334, and 336 can be turnedoff. There is thus a delay in turning the switches 308, 312, and 316 offand turning the return switches 332, 334, and 336 on. The energy storedin the inductive connection 330 is then added to the energy beingdischarged from the capacitors 210, 314, and 318. Combining theinductive energy of the inductive connection 330 with the capacitiveenergy stored in the capacitors 310, 314, and 318 results in a fasterrise time of the voltage pulse. One of skill in the art, however, canappreciate that an inductive connection does not require a delay to beincorporated between turning the switches 308, 312, and 316 to an onstate and turning the return switches 332, 334, and 336 to an off state.

When the pulse generator is ready to terminate the high voltage pulse,the switches 308, 312, and 316 are typically turned off. The fall timeof the high voltage pulse can be improved by turning on the returnswitches 332, 334, and 336. Opening the path through the return switchescan help discharge stray capacitance and/or load capacitance, whichimproves the fall time of the high voltage pulse.

This example illustrates that the timing used to control the mainswitches 308, 312, and 316 and of the return switches 332, 334, and 336can be used to control or alter the rise time and/or the fall time ofthe resulting voltage pulse. The shape of the voltage pulse can also beprogrammed in some embodiments.

FIG. 4 illustrates a block diagram of a pulse generator that includesmultiple voltage cells (also referred to herein as stages or sections).The example of the pulse generator illustrated in FIG. 4 generates apositive voltage pulse. FIG. 4 illustrates the voltage cells 474, 472,470, and 468 that are connected as previously described using mainswitches 414, 416, 418, and 420 controlled by switch drives 448, 452,456, and 460, and return switches 438, 440, 442, and 444 controlled byswitch drives 446, 450, 454, and 458. In this example, the return pathfrom the supply line 404 includes inductive connections 415, 417, and419 from the charging capacitors 422, 424, 426, and 426 through thereturn switches.

FIG. 4 further illustrates an auxiliary path 473 that is used by thepower supply 466 to provide power to the switch drives 446, 448, 450,452, 454, 456, 458, and 460 (446-460). The auxiliary path 472 includesthe auxiliary diodes 476, 478, 480, and 482 (476-482). The auxiliarydiodes 476-482 help isolate the power supply 466 and help deliver pulseto the load 402.

The auxiliary diode string that includes the auxiliary diodes 476-482represent a voltage drop for each diode in the diode string. Thus, thevoltage available at a particular stage may be affected by the forwardvoltage drops of the diodes in the diode string. The voltage provided bythe auxiliary power 466 simply provides sufficient voltage to overcomethe forward voltage drops of the diodes and/or the charging switchvoltage drops. If a large number of voltage cells are included, boostingvoltage supplies may be included to provide adequate voltage levels.

The switch drives or switches 446-460, in one embodiment, can be anytype of solid state switches known in the art. Bipolar junctiontransistors, field effect transistors, IGBTs, Darlington Bipolartransistor, solid state switches, and the like are examples of switchesthat can be used as described herein. Each voltage cell includes aswitch drive for a main switch and a switch drive for a return switch.For example, the voltage cell 468 includes a switch drive 448 used tocontrol the main switch 414. In this example, the gate of the mainswitch 414 is controlled by the switch drive 448. The switch drive 446controls a state of the return switch 438.

The voltage available to the switch drives 446-460 is often reduced atsuccessive switch drives by the voltage drop across previous diodes inthe diode string and switches. Each switch drive can be driven fromeither ground or from the previous voltage cell. In one embodiment,DC-DC converters may be used to provide adequate voltage. In anotherembodiment, the switch drives are optically coupled from ground.

The energy storage capacitors 422, 424, 426, and 428 are charged by wayof the diodes 406, 408, 410, and 412 and the return switches. Chargingthe capacitors in this manner eliminates the use of inductors,resistors, or isolated supplies that are common in conventional MarxGenerators. In addition, the energy needed to drive the switches canalso be provided through the diode string in the auxiliary path 473,eliminating the use of inductors, resistors, or isolated supplies orstep down supplies that may otherwise be needed. The switches can betriggered by way of example, fiber optic coupling, transformer coupling,or by the auxiliary power diodes.

The diode string that includes the diodes 406, 408, 410, and 412provides several advantages. First, the diode string isolates eachvoltage cell or voltage stage from other voltage cells or stages duringthe pulse. The diode string also an alternate current path around aparticular voltage cell or stage of the switch for that particularvoltage cell is not turned on or is delayed. The diode string enables avoltage pulse to be delivered even though a voltage cell is delayed orfails.

FIG. 4 further illustrates balance networks 430, 432, 434, and 436. Eachbalance network typically includes a capacitor in series with a resistorand each balance network helps balance the stray capacitance to ground.The capacitance in the balance networks helps to equally distribute thevoltages from section to section during the rise time and the fall timeof the voltage pulse. Because the stray capacitance to ground associatedwith a particular voltage cell is typically different from the straycapacitance to ground associated with other voltage cells of the pulsetransformer, the capacitance and/or resistance of each voltage cell canbe adapted to match the stray capacitance “seen” by that voltage cell.Thus the capacitance of the balance network 430 may be different fromthe capacitance of the balance networks 432, 434, and 436. Thecapacitance of each balance network is selected to match the straycapacitance. The resistance in each balance network helps reduce ringingof the stray inductance and/or the stray capacitance. In an alternativeembodiment as shown in FIG. 8, a ringing circuit may be included in oneor more of the voltage cells of a pulse generator to provide at least asmooth droop compensation to the voltage pulse.

The power supply 462 can provide a source of power at the high voltageend of the load 402. For example, if the load 402 is a pulsed tube, thenthe power supply 462 can provide power for the filament or heater of thepulsed tube. Thus power supply 462 provides a power source at the highvoltage end without additional equipment.

FIG. 5 illustrates another embodiment of a pulse generator. FIG. 5 issimilar to the pulse generator illustrated in FIG. 4, with thedifference that the pulse generator in FIG. 5 generates a negativevoltage pulse whereas the pulse generator of FIG. 4 generates a positivepulse. The charging diodes 502, 504, 506, 508, and 510 and the auxiliarydiodes 512, 514, 516, and 518 are configured to accommodate a negativepower supply 500, 520. The switches and the return switches are alsoadapted to a negative supply.

FIG. 6 illustrates an embodiment of a pulse generator that has a bipolaroutput. In other words, the pulse generator 600 illustrated in FIG. 6can generate both positive and negative type voltage pulses. The bipolarpulses can be generated by stacking voltage cells configured to generatea positive voltage pulse in series with voltage cells configured togenerate a negative voltage pulse.

In FIG. 6 the positive voltage cells 618 generate a positive typevoltage pulse and the negative voltage cells 620 generate a negativetype voltage pulse. The voltage cells 618 are in series with the voltagecells 620. In this example, the diode string 602, which is used tocharge the capacitors in the voltage cells 618, is connected with thereturn line switch string 604 of the voltage cells 620 via theconnection 606. Similarly, the diode string 610, which is used to chargethe capacitors in the voltage cells 620, is connected in series with thereturn line switch string 622 of the voltage cells 618 via theconnection 608. The negative supply auxiliary diode string 614 isconnected with the positive supply auxiliary diode string 616 using aninverting DC-DC supply 612. All of the capacitors in the positivevoltage cells 618 and the negative voltage cells 620 can be charged atthe same time.

FIG. 7 illustrates an embodiment of a pulse generator that includesdroop correction. More particularly, FIG. 7 illustrates droop correctionfor a negative type pulse generator. The embodiment of the pulsegenerator illustrated in FIG. 7 includes a plurality of voltage cells714 as previously described. In this example, the voltage cells 714 areconnected with a series of cells 702 that are different from the voltagecells 714.

In this example, the voltage cells 702 are configured such that theycharge in series and discharge in parallel. The switch drives 708, 716,718, 720, and 722, control the switches 706, 732, 734, 736, and 738 suchthat the capacitors 704, 724, 726, 728, 730, and 740 charge in series.At the same time, the storage capacitors of the voltage cells 714 arecharging in parallel. However, the voltage cells 702 are configured toprovide droop correction.

When the switches in the voltage cells 702 are on, the capacitors chargein series. During the voltage pulse, the voltage cells 702 can bedischarged such that the shape of the voltage pulse can be adjusted. Inone embodiment, the droop can be corrected across the entire pulse bycontrolling or delaying the discharge of the capacitors in the voltagecells 702.

In another embodiment of the invention, the pulse may drive a pulsetransformer with a core that needs to be reset. A reset supply could beincluded in series with the ground end of the charging switch to providethe core reset current. This eliminates the need to have a core resetinductor.

FIGS. 8A through 8D illustrate additional embodiments of a voltage cellincluding circuit that provides droop compensation. FIGS. 8A through 8Dalso illustrate the ability to isolate a defective or non-functioningvoltage cell. When a pulse generator is constructed, it may include aplurality of voltage cells. Embodiments of the invention provide droopcompensation by including a ringing circuit. However, only one or a fewof these voltage cells need to have a ringing circuit in order toprovide droop correction or droop compensation. The majority of thevoltage cells can be as described herein and do not necessarily need toinclude the ringing circuit.

In this example, the capacitors 802 are used to store the charge that isdelivered to a load. Other capacitors in the serially connected voltagecells also store charge that will be delivered to the load as a voltagepulse. As previously described, the capacitors 802 can be charged inseries as the current flows from the charging source 824 through thediode 836. The current used to charge the capacitor(s) in the voltagecell 820 flows from the charging power 824 through the return switch814, which is on, and up to the voltage cell 820 and then back throughthe diode string 822 to the charging power 824.

In this manner, the capacitors 802 are charged, as illustrated in FIG.8A, when the main switch 816 is off and the return switch 814 is turnedon. When the return switch is turned on in this manner, current can flowto the capacitors in the next voltage cell 820. Because the mainswitches in the series connected voltage cells are turned off, thecapacitors 802 do not discharge. The current through the return switch814 can return through the diode string 822. In this manner, thecapacitors in the voltage cells are charged effectively in parallel bythe charging power supply 824.

The voltage cell 800 in FIG. 8A also illustrate an inductor 812, acapacitor 810, and diodes 804, 806, and 818. The diodes 818 and 806prevent the capacitors 802 from discharging. When the main switch 816 isturned on, the diode 804 prevents the current stored in the capacitorsfrom discharging through the capacitor 810 and insures that the voltagepulse is delivered to the load.

As previously indicated, a voltage pulse may begin to droop over time.The ringing circuit that includes the capacitor 810 and the inductor 812can provide droop compensation that is smoother than the droopcompensation illustrated in FIG. 7. In this case, as the capacitors 802begin to discharge, the ringing circuit begins to ring, creating a halfsine wave in one embodiment that provides droop compensation to thevoltage pulse. Advantageously, this provides smooth droop compensationin one embodiment rather than jagged or saw tooth compensation to thevoltage pulse.

More specifically, FIG. 8B illustrates a voltage cell that isdischarging and providing droop compensation. In this example, the mainswitch 816 is turned on and the return switch 814 is turned off. Thus,the capacitors 802 of the serially connected voltage cells 800, 820(additional voltage cells may be similarly connected as previouslydescribed) are connected and are discharging through the open switchesto the connected load, which may be in parallel to the seriallyconnected voltage cells.

In a voltage cell that includes a ringing circuit, the diodes 842, 836,and 840 enable the capacitors 802 to charge in series and also cause thecapacitors to discharge in parallel. Although the capacitors 802 in thecell 800 charge in series, the capacitors 802 are charging in parallelwith other capacitors in other voltage cells as previously described.

As a result, the voltage cell 800 is connected with the ringing circuitsuch that when the switch 816 is turned on, the capacitors have half ofthe voltage. The ringing circuit begins to ring and the capacitor 810 ischarged to twice the voltage. The capacitor 810 thus charges in a sinewave and provides smooth boost compensation to the voltage pulse. Inthis example, the circuit 838 includes diodes and resistors that providea path for the charge to discharge in parallel.

The droop compensation or droop correction is provided by the ringingcircuit. In this example, the inductor 812 and the capacitor 801 beginto ring and as the capacitor 810 begins to charge, it can dischargethrough the switch 816, which is on, and to the load. The ringing hasthe effect of providing droop compensation to the voltage pulse. Byvarying the values of the capacitor 810 and of the inductor 812, thedroop compensation can be selectively provided and controlled. In otherwords, the capacitor 810 begins discharging after the capacitors inother voltage cells are discharging, thereby compensating for voltagedroop in the voltage pulse.

FIGS. 8C and 8D illustrate the ringing circuit after the main switch 816is turned off after the pulse is provided. When the main switch 816 isturned off (along with the main switches in other voltage cells), theringing circuits of the voltage cells become isolated. The residualcurrent in the inductor 812 or charge in the capacitor 810 discharges inthe ringing circuit in an isolated manner. FIG. 8C, for example,illustrates that the current 828 discharges from the capacitor 810through the diode 818. FIG. 8D, on the other hand, illustrates that thecurrent in the inductor 812 discharges to the capacitor 810 through thediode 806. In this manner, the ringing circuit illustrated in FIGS. 8Athrough 8D is an example of providing smooth droop compensation to avoltage pulse.

FIG. 8D also illustrates an embodiment of the invention that can isolatea defective cell or isolate a cell that may not be functioning correctlywithout having an adverse impact on the generation of a voltage pulse.In this case, the return switch illustrated in FIG. 8D (and in otherFigures) includes a diode 832. If the return switch does not includesuch a diode, the diode 834 can be included in the voltage cell acrossthe return switch in this example. The diode 832 provides a path for thedischarge current when the return switch 814 is off and the cell isdefective. Thus, the diode 832 or 834 becomes an isolation diode thatenables the pulse to be delivered when a particular voltage cell isinoperative for various reasons.

Usually, when the voltage cell 800 is operating normally, the diode 832or 834 is reverse biased during discharge such that current dischargesthrough the capacitors. When the cell is defective, the switch 816 canbe turned off and this enables the discharge current to pass around thedefective voltage cell through the diode 832 or 834.

When the return switch is on to permit charging of the capacitors 802,the main switches are off and the current is prevented from dischargingthrough the diode 832 because the voltage cells are effectively isolatedwhen the main switches are off and the diode 832 is reversed biased.Alternatively, the discharge current can be delivered through the diodestring 822.

In one embodiment, a pulse generator that includes multiple voltagecells can be constructed. Such a pulse generator has multiple redundancythat provides protection for several events. If a voltage cell isdefective, the amount of charge initially stored in the capacitors ofthe remaining voltage cells can be altered such that the voltage pulseis not affected. In this case, the pulse generator may include morevoltage cells than are required for a particular duty. Also, the voltagecells are constructed in a manner that permits the voltage pulse to bedelivered even when a cell is defective. In other words, embodiments ofthe invention provide cell isolation and redundancy.

Embodiments of the invention provide several advantages and benefits.The shape of the pulse as well as the rise time and the fall time can beprogrammed or controlled. For example, the rise time and fall time canbe controlled by selection of the components in the ringing circuit. Therise time and/or the fall time can also be controlled by timing when themain switches and/or the return switches are turned on/off. In additionthe length of the pulse can also be programmed.

For example, a pulse generator typically includes enough voltage cellsto generate a wide range of voltage pulses. In some instances, not allof the voltage cells may be needed to generate a particular pulse. Bycontrolling the timing of the main switches and the return switches cancontrol the length of the voltage pulse. In another example, by turningthe return switches on at the end of the pulse, the fall time of thevoltage pulse can increase because any residual voltage has another pathin which discharge can occur.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A pulse generator that generates a voltage pulse that is applied to aload, the pulse generator comprising: a plurality of first voltagecells, each voltage cell having a capacitor in series with a main switchsuch that all capacitors are connected in series when all main switchesare on to generate a voltage pulse that is delivered to a load; and asecond voltage cell connected in series with the plurality of firstvoltage cells, the second voltage cell including: a pair of capacitorsin series with a second main switch; and a ringing circuit that providesdroop compensation to the voltage pulse.
 2. A pulse generator as definedin claim 1, wherein the pair of capacitors are connected such that thepair of capacitors charge in series with the capacitors in the pluralityof first voltage cells and discharge in parallel to the ringing circuit.3. A pulse generator as defined in claim 2, wherein the ringing circuitincludes: an inductor in series with the pair of capacitors; and aringing capacitor in parallel with the inductor, wherein the ringingcapacitor charges to twice a value of the pair of capacitors andprovides droop compensation to the voltage pulse.
 4. A pulse generatoras defined in claim 1, wherein the plurality of first voltage cells eachcomprise a return switch.
 5. A pulse generator as defined in claim 4,wherein a charging source provides a charging current that passesthrough each of the return switches to charge the capacitors in theplurality of first voltage cells.
 6. A pulse generator as defined inclaim 5, further comprising a diode string that provides a return pathto the charging source for the charging current.
 7. A pulse generator asdefined in claim 1, wherein the second voltage cell further comprises afirst diode between the pair of capacitors, wherein the first diodeallows a charging current to charge the pair of capacitors in series andcauses the pair of capacitors to discharge in plurality.
 8. A pulsegenerator as defined in claim 1, the second voltage cell furthercomprising a second return switch.
 9. A pulse generator as defined inclaim 8, wherein control signals are used to turn the main switches andthe return switches on or off in order to control a rise time or a falltime of the voltage pulse.
 10. A pulse generator as defined in claim 1,wherein the ringing circuit provides smooth compensation by generating ahalf sine wave voltage output.
 11. A pulse generator that generates avoltage pulse that is applied to a load, the pulse generator comprising:a main switch; a first capacitor in series with the main switch; asecond capacitor in series with the first capacitor; a first diodeconnected between the first and second capacitor, wherein a chargingcurrent charges the first and second capacitor in series; a ringingcircuit connected to the first and second capacitor, the ringing circuitincluding: a first inductor in series with the first and secondcapacitor; and a third capacitor across the first and second capacitor;and a plurality of diodes arranged with the first and second capacitorsuch that the first and second capacitor charge in series when the mainswitch is off and discharge in parallel when the main switch is on,wherein the ringing circuit generates a voltage that is twice thevoltage of the discharging first and second capacitor and that providessmooth droop compensation to a voltage pulse.
 12. A pulse generator asdefined in claim 11, further comprising a return switch, wherein thecharging current passes from a charging source through the return switchand charges a capacitor of another voltage cell.
 13. A pulse generatoras defined in claim 12, further comprising a first switch drive thatcontrols the main switch and a second switch drive that controls thereturn switch.
 14. A pulse generator as defined in claim 11, furthercomprising a plurality of voltage cells connected in series with thefirst and second capacitor, wherein the plurality of voltage cells eachhave a capacitor that receives a charging current through a returnswitch of another voltage cell.
 15. A pulse generator as defined inclaim 11, wherein the main switch and the return switch are controlledwith control signals in a manner that permits a rise time of the voltageand a fall time of the voltage pulse to be controlled.
 16. A pulsegenerator as defined in claim 15, wherein the return switch is turned onto increase a fall time of the voltage pulse by allowing charge todissipate through the return switch.
 17. A pulse generator as defined inclaim 15, wherein the main switch is powered thorough an auxiliary diodestring and wherein the control signals are optically coupled to the mainswitch and to the return switch.
 18. A voltage pulse generator forgenerating a voltage pulse, the voltage pulse generator comprising: aplurality of first voltage cells each having a main switch and acapacitor in series such that each main switch and each capacitor is inseries with other main switches and other capacitors in other firstvoltage cells when the main switches are on, wherein the capacitors areisolated from each other until the main switches are turned on such thatthe serially connected capacitors deliver a voltage pulse without usingtransformers; a diode string connected with the plurality of firstvoltage cells; a plurality of first return switches included in theplurality of first voltage cells, wherein a charging current passesthrough the first return switches and the diode string such that thecapacitors in the plurality of first voltage cells are charged inparallel; and a second voltage cell that provides droop compensation tothe voltage pulse by generating a half sine wave voltage using a ringingcircuit.
 19. A voltage pulse generator as defined in claim 18, whereinthe second cell comprises: a second main switch; at least a pair ofcapacitors in series with the main switch and connected using aplurality of diodes such that the pair of capacitors charge in seriesand such that the pair of capacitors charge in parallel with respect tothe capacitors in the plurality of first voltage cells, wherein theplurality of diodes are arranged such that the pair of capacitorsdischarge in parallel when the second main switch is on; and a ringingcircuit connected to the pair of capacitors, wherein the ringing circuitrings when the pair of capacitors discharge to generate a sine wavevoltage that provides droop compensation to the voltage pulse, theringing circuit including an inductor in series with the pair ofcapacitors and a third capacitor across the pair of capacitors.
 20. Avoltage pulse generator as defined in claim 19, further comprisingringing diodes that are connected between the ringing circuit and thepair of diodes such that energy in the inductor or in the thirdcapacitor can discharge into the pair of capacitors when the main switchis turned off.